Particle decay lifetimes

I'm revising for an exam on particle physics and understand the lot, however I'd like clarification on the calculation of lifetimes of particles.

I understand that particles decaying by the strong interaction last roughly 10^-23s because that is the minimum time that information can cross a nucleus of order a fermi (ie at light speed). Also I know that EM decays take about 10^-16 seconds (like in the pi-zero decay to 2 photons), but I'd like to know why? Also why does the weak decay take about 10^-10s? (give or take a few orders of magnitude!)

One could expect EM decays in general to be as faster as strong decays, shouln't one? but the ones you have mentioned are very special ones, they are impossible due to chiral considerations (which I do not remember) and they happen because of the chiral anomaly.

Although the weak nuclear force used to be described by Fermi's theory of a contact four-fermion interaction, today we know that it is mediated by the W and Z bosons. Because of their large mass of about 90 GeV/c2, their mean life is limited to about 3 * 10^{-25}seconds
Hope that is helpfull, and sorry about my english.

EM decays usually have a factor of \alpha^2_EM (=1/137) in calculating their rate. This makes them about 10^4-10^5 slower than strong decays.
Weak decays have another factor of (M_p/M_W)^2 [~(1/80)^2],
which makes them slowest of all.